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11    THE SPACES L1 AND L2       171

for some integer k between 0 and n  1 ; show that         fdp ^ 0 by using (c) above.)

J Ejj(,f)

Hence deduce the maximal ergodic theorem of Section 13.9, Problem 12.

17. Suppose that U satisfies the conditions of Problem 16 and also that for each / > 0 the
relation I/O) | < t almost everywhere in X implies that | (U /)(*) | rg / almost every-
where. (This will be the case whenever ^ is bounded and U - c = c for each constant c.)

(a)    Show that, for each function /e & and each t > 0,

(1)                            (U-f-t)+<U*(f~ty

almost everywhere. (Consider the function which is equal to /when \f(x)\ */, to t
when f(x) > r, and to t when f(x) < --/.)

(b)    Show that, for each function fe&^n&g, the function U / belongs to ^
for 1 <: p <s + oo and that

(Deduce from (1 3.21 .9) that for each/e & n JSfff , the function (*, /) >-> tp~2(f(x) - 1)+
is integrable relative to the measure on X x ]0, + oo [ which is the product of /x
and Lebesgue measure, and then integrate the inequality

tp'2(u /- o+ < ^-2(/- /)+.)

Deduce that (/can be extended to an endomorphism of norm ^ 1 of each of the spaces
J&& . Denote this extension also by U.

(c) Let /e J$?& for some p  [1, + oo[. For each t > 0 and each integer n J> 0, let
E,,,t(/) denote the set of points x e X such that

/(*) + / ' /)(*) ~l~ - '  4- (Un- 1  f)(x) > nt,

and let Et(/) denote the union of the EM,r(/). Show that each of the sets En,t(f) is
integrable and that

(Observe that EM(/) is contained in the set of points x e X satisfying one or other
of the n relations (Uk  f)(x) > t, where 0 < k ^ n  1 ; then continue as in Problem 16,
by remarking that

(U-f+U2-f+--- + Uk-l'f-(k + W

g U- (/H- (//+ + Uk f-(k+l)t)+.)

(d)   If /e J2?i , deduce from (c) that

for all / > 0.
18.   Suppose that U satisfies the conditions of Problem 16. Let f,g e Jf be such that

(a)   Let  be a real number > 0, and let An be the set of x e X which satisfy the
inequality

(Un /)(*) > e(ff(x) + (U- g)(x) -f    -f (Un^r/ 1) /)->0 as t-*Q. (Let /== u + w where u and v are real; observe